1. Field of the Invention
The invention relates to the field of radio-frequency engineering. It relates in particular to a tunable radio-frequency filter arrangement and to a method for its production.
A radio-frequency filter arrangement of this type is known, for example, from U.S. Pat. No. 6,147,577.
A single tunable dielectric resonator, in which the moving dielectric body can move linearly in the vertical or horizontal direction in a cutout in the dielectric resonator element is known, by way of example, from EP-A1-0 601 369.
2. Description of the Related Art
Transportable radio link connections (LOS=Line of Sight) have been proven for rapid and flexible construction of wire-free communication networks, in particular in rugged terrain without a suitable infrastructure, and these operate in the frequency range of two or more GHz (for example 4.4 to 5 GHz; or 14.62 to 15.23 GHz). Appropriate filters, in particular bandpass filters, are required for signal processing within the transmission and reception arrangements for such directional radio links, which filters are designed not only for individual frequencies but are automatically tunable and are distinguished by constant high Q-factors over the tuning range.
In addition to the essential electrical and radio-frequency characteristics, filters of these type must, however, also be producible at low cost, must have a robust design, and must be designed to be reliable in use and to be compact and light-weight. Space (volume) and weight, in particular, are major factors for the mobility of the overall communication system.
In the past, in order to reduce the size of the cavities for filters of these type, solutions have increasingly been proposed which have a dielectric resonator element arranged in a cavity as the tunable basic element, whose resonant configuration can be varied in order to tune the filter. One such solution is described, by way of example, in U.S. Pat. No. 6,147,577, which was initially cited. In this known solution, a first round dielectric disk (ceramic puck) is arranged in a fixed position as a resonator in each of the cavities of the filter. An identical second round dielectric disk is located parallel above the first, and can be raised vertically, and lowered again, relative to the first disk by means of an electronically controlled motor drive. The linear movement that is required for this purpose is produced by a digital stepping motor, whose rotary movement is converted to a linear movement by a complex threaded rod mechanism.
This known filter arrangement has various disadvantages: firstly, it is comparatively difficult to achieve the comparatively high accuracy and reproducibility of the disk position during a linear movement of the moveable disk, as is required for good tunability of the filter. Secondly, the adjustment mechanism that is required for the linear movement requires a very large amount of space. As can easily be seen from FIG. 4 in U.S. Pat. No. 6,147,577, the motorized adjustment mechanism that is arranged above the cavities occupies about ⅔ of the entire physical volume of the filter. Furthermore, due to the capability of the upper disk to move in the vertical direction, the cavity must be initially designed to be comparatively large.
EP-A1-0 601 369, which was likewise cited initially, proposes a single tunable dielectric resonator in which an eccentric cutout is provided in the dielectric disk that is arranged in a fixed position in a cavity, which cutout can be entered to a greater or lesser extent by a dielectric body that is shaped to match the cutout. The resonator is tuned by adjustment of the insertion depth. For this purpose, the dielectric body can be moved linearly via a holder in the form of a rod in the vertical direction (FIG. 1 in EP-A1-0 601 369) or in the horizontal direction (FIG. 2 in EP-A1-0 601 369). No further details are stated about the tuning response that can be achieved by this solution. Furthermore, no mechanically adjustment mechanism is specified either, so that this proposal should in fact be regarded just as paper prior art, and its feasibility is more than questionable. In particular, this solution proposal is also subject to the same disadvantages resulting from the linear movement as those which have already been discussed further above.